Change "new" to "modified"

Author: jtraglia

specs/electra/beacon-chain.md

@@ -298,15 +298,15 @@ class Attestation(Container):
 ```python
 class AggregateAndProof(Container):
     aggregator_index: ValidatorIndex
-    aggregate: Attestation  # [New in Electra:EIP7549]
+    aggregate: Attestation  # [Modified in Electra:EIP7549]
     selection_proof: BLSSignature
 ```
 
 #### `SignedAggregateAndProof`
 
 ```python
 class SignedAggregateAndProof(Container):
-    message: AggregateAndProof   # [New in Electra:EIP7549]
+    message: AggregateAndProof   # [Modified in Electra:EIP7549]
     signature: BLSSignature
 ```